The need for steerable auxiliary axles on various types of vehicles (trucks, trailers, etc.) is well known. By the term "auxiliary" axle, as used herein, is meant an axle which is not the axle primarily used for steering the vehicle. Instead, an "auxiliary" axle is one usually added to the vehicle so as to be either forward or rearward of the primary axles and rearward of the vehicle's primary steering axle (e.g. in a straight truck such as a dump truck, garbage truck, etc.). The purpose of adding such an auxiliary axle to the vehicle is normally to increase the load carrying capacity of the vehicle and/or to satisfy highway weight limit laws. Such auxiliary axles may be of the non-liftable or liftable type as contemplated herein.
To render a wheeled axle bearing suspension system "steerable", the caster angle (from the vertical) of the axle (or as may be used at times synonymously in the art, the caster angle of the wheels) must be appropriately chosen to make the wheels of the suspension steerable. A recognized, but not necessarily required, caster angle (from the vertical) is from about 3.degree. to 5.degree.. Thus a 5.degree. caster angle, for example, may be employed for steerable forward travel, while for reverse travel the caster must then be changed, for example, to a -3.degree. caster angle from the vertical. In short, a caster angle for steerability in one travel mode does not normally render steerable the wheel in the opposite travel mode. A need thus arises in the art to provide a mechanism for adjusting (changing) the caster angle, thereby to allow the axle (e.g. auxiliary steerable axle) to be steered during either the reverse or forward mode of travel.
One known and rather successful system has been developed to provide an adjustable mechanism whereby the vehicle (e.g. heavy duty truck or semitrailer) operator can change the caster angle of his auxiliary axle. This mechanism is illustrated in U.S. Pat. No. 6,007,078 and constitutes a principal background from which this invention arose. As to certain embodiments of this invention, they may be considered specific improvements upon this aforesaid patent in that there is herein provided a unique mechanism for adjusting the caster angle of a wheeled axle which may be used instead of the caster angle adjustment disclosed in this prior patent. The same basic parallelogram structure, however, is otherwise maintained so as to couple its unique efficacy with that of this invention. For this reason, the entire disclosure of U.S. Pat. No. 6,007,078 is herein incorporated by reference.
Generally speaking, in order to achieve the desired forward and rearward steerability in the now known parallelogram suspension system disclosed in the '078 patent, there is employed a pair of opposing air brake chambers each including an extendible and retractable rod. In this prior art system, one end of each rod is connected to a cradle which is further connected at a pivot to a hanger bracket which, in turn, is secured to the frame of the vehicle. In addition, two arms connect the cradle to an axle seat (retaining a wheeled axle) thereby forming the basic parallelogram structure. Each respective arm is connected to both the cradle and the axle seat by pivots. Thus, when one of the opposing air brake chambers is pressurized, and the opposite chamber is simultaneously exhausted of air, the reciprocating rods extend or retract in respective fashion, thereby causing the cradle to pivot with respect to the hanger bracket. As the cradle pivots with respect to the hanger bracket, the axle seat is simultaneously pivoted (via the two arms), thereby changing the caster angle of the wheeled axle.
Although the aforesaid system is both successful and effective, it is desirable in the art to achieve a steerable suspension system of decreased complexity and reduced size which may be produced and maintained at a more attractive cost. In particular, it is desirable to achieve a suspension system which maintains steerability while eliminating unnecessary external mechanical forces.
It is a purpose of this invention to fulfill this and other needs which will become more apparent to the skilled artisan once given the following disclosure.